Cadmium sulfide barrier layer cell



D. C. REYNOLDS CADIUMv SULFIDE BARRIER LAYER CELL July 2z, 195s 2,844,640

Filed May 11. 1956 2 Sheets- Sheet 1 NE' TRL Y :Pace :A1/nece NMi- Mae/61e Oooooooo M571( /C comaucme /V- 7 YH!" 56W/ O o/vaucrae P- WP; sewn/mueras .El b fria/c ramo/fc rae III! IN VEN TOR. aow/ua 6. .e5 Y/vua s www July 22, 1958 D; c.` REYNOLDS 2,844,640

- CADIUM SULFIDE BARRIER LAYER CELL] Filed May 11. 195e 2 sheets-sheet 2 *ETE-i o INVENTOR. 00A/ma e @fm/0105 United States Patent() .cADMmM sULEmE BARRIER LAYER CELL Donald C.Reynolds, Springfield, Ohio, assignor to the United States of America as represented by the Secretary of the Air Force Application `Mayll, 1956, Serial No. 584,403

4 Claims. (Cl. 13G-89) (Granted under Title-35, U. S. Code (1952), sec. 266) The invention described herein may be manufactured and used by or for the United States Government for governmental purposes without payment to me of any royalty thereon.

This invention relates to a cadmium sulfide barrier layer cell excitable by photons and other types of ionizing radiation.,

As a background for insuring a suiiicient understanding'of the present invention `as claimed, solid materials are made up of atoms. All atoms have electron shells. Each electron in the various shells of an atom has a -discrete electrical energylrelative to the `atomic: nucleus. When a large group ofv like 4atoms are brought together in a solid, the `electron .shells of the latoms forman energy band structure. The electrical properties of pure or ideal materials are determined by the relationship of thebands formed from the outermost shell.

Illustrative diagrams are shown in Figs. 1 and 2 of the accompanying drawings wherein Fig. l illustrates four types of band structures possible among pure or ideal materials andFig. 2 represents potential energy curves applicable to the theory of operation of the present invention.

If the band designated valence band inFig. la, formed from the valence shell of an atom does not haveits full quota of` electrons, `a material of the -type shown in Fig. 'la is obtained. Ifthis band is full of electrons, but the next band overlaps it, one obtains -a material *of the type shown in Fig. lb wherein notorbidden energy gap appears. These Fig. laan-d'Figlb materials are metallic conductors with the Fermi level at the 4top of the iilled level. However, if 'the band formed in the .outermost shell of an atom is full andthe next allowed band does not overlap, Aa forbidden energy region or gap exists between the full and -the empty bands, as indicated inFigs. 1a, 1c or ld of the drawings. .These materials. are either semiconductors Fig. 1c or insulators Fig. ld, depending :on how large the forbidden gapis.

The invention Vherein directly concerns semiconductors represented in Fig. lc lof the drawings. The forbidden gap in. these semiconductor materials is small enough so that at room temperature there willbe some electrons excited to the conduction band. An electric field lapplied to such a material willcause some conduction.

These .materials are intrinsic semiconductors and 'the conductivity consists of the combined movements of electrons and of positive holes.

Itis usually lpossible to obtain solid solutions of impurities and disorders in the lattice structures .of these materials. The energy levels of the impuritiesusually fall in the forbidden region or forbidden energy gap. Electronegative impurity atoms donate electrons and generally have an electronic .energy level lying close tothe empty'vband or conduction band. Electropositive impurity atoms accept electrons and generally have elec-v tronic energy levels lying close to the full band of the matrix material. These are extrinsic semiconductorsand Patented July 22, 1958 ICC will be either N-type or P-type depending .on .whether they have electronegative or electropositive impurity atoms respectively incorporated in their crystal lattice. The Fermi level in these materials will in general be 'between the impurity level and the conduction band in an N-type material and between the impurity level and the valence band-in `al P-type material.

When .a metallic conductor anda semiconductor, or when two semiconductors are brought into contact, a rectifyingbarrier, such `as is shown in Fig. 2, Willbe formed if certain conditions are met: (a) when a metal and an N-'typesemi-conductor are brought into contact, if the work function of the metal is greater than that of the semiconductor; (b) when a metal and a P-type semiconductor are brought into contact, if the work function of the metal is less than that of the semiconductor; and (c) when a'P-type semiconductor is brought into contact with an N-type semiconductor, if the metal work function is equal to or is greater than the semiconductor work function, there will be 'no barrier. In Fig. 2 energy is alongthe ordinate and distance is along the abscissa.

The'term work-function is defined at page 85 of An Introduction to Semiconductors by W. C. Dunlap, Jr., published in 1957 by John Wiley & Sons, Inc., New York city,'New York, as the energy required to take an electron'from the'Fermi level out through the surface to infinity. Introduction to Solid State Physics by Charles Kittel, published in 1954 by John Wiley & Sons, Inc..on page 236 defines work function -as the work necessary to remove-to infinity an electron from the lowest free electron state in the metal. A second edition of the Kittel Work Was published in .1956 wherein the definition was repeated on page 266 and developed further at page 387 and elsewhere.

When hole-electron pairs are created in the barrier region, or if they can diffuse to the barrier regio-n, they will be in the iniiuence of the barrier layer tieldwith the electron being idrawn to the positive space charge and the hole -being drawn to the negative space charge, in which situation Ielectrical energy may be drawn away from the cell.

When these hole-electron pairs are created by light 'it it calledl a photovoltaic cell. They may also be created byvarious types of nuclear radiation, such as electrons, gamma rays, X-rays, etc.

A 'In most cells the photons creating thef-hole-electron pairs must have enough energy to excite an electron across the forbidden energy gap. -In cadmium sulfide cells this isnot necessary, ythe photovoltaic eect can be obtained from impurity levels as Well as the intrinsic level, where the electron is'excited across the gap.

Arepresentative publication which elaborates further onthe above theory is Electrons and Holes in'Semiconductors writtenby William Shockley and published in 1955 by the D. Van Nostrand Company, Inc. of New York city, New York.

A general statement of the nature of the present invention `as claimed is vthe conversion of radiation energy into electrical energy. The substance of the present invention is Ithe provision of a photovoltaic cell which converts radiation energy into electricity.

-A general-.object of the .present invention as claimed is to provide a means and a method for converting energy from thersun into electrical energy.

vCacrnium'fsuliide barrier layer cells which embody the present invention and a graph record of the performance of one of the cells as. compared with solar energy are represented intheaccompanying drawings wherein:

Fig. 3'is.a perspective elevational view of a cadmium sulfide barrier. layer cell with a polished face radiated by the suns energy;

Fig. 4 is a sectionttakenfalong the1ine'4-4 of Fig. 3;'

Fig. 5 is a section of an N-type cadmium sulfide cell v cadmium sulfide cell performance curve superimposed upon each other plotted on coordinates of wave lengths along the abscissa and current along the ordinate.

The cadmium sulfide barrier layer cell which is represented in Figs. 3 and 4 of the accompanying drawings is a cadmium sulde crystal with an N-type side 11 and a P-type side 12 between which sides is a barrier layer 13 across which electrons flow during -the radiation of the crystal with the application to the cell of the full solar spectrum of energy from the sun 14. The crystal side which is to be radiated with energy from the sun is polished to a desired degree for optimum light penetration.

Suitable electrical contacts are applied to the crystal illustratively along each of its opposite surfaces just inwardly from the edges thereof by means suitable for the composition of the contacts, such as by electrode plating, melting, painting, thermo deposition by evaporation or the like. The contacts `are as narrow as possible to expose the maximum area of the polished cadmium sulfide v crystal to the action of the suns energy. Transparent contacts are within the concept of the present invention.

In selecting a contact material for application to a cadmium sulfide crystal, the relation between its work function and the work function of the cadmium sulde crystal is taken into consideration as will be presented more explicitly hereinafter.

Contacts applied to the opposite sides of the cadmium sulfide crystal may be classied as and are referred to herein as ohmic contacts and as rectifying contacts. Most metals conduct electrical energy more eiciently in one direction than in the other. Methods of making contact to semiconductors and to ohmic or rectifying contacts are described at page 192 in the Dunlap text. The International Dictionary of Physics and Electronics, published in 1956 by D. Van Nostrand Company, Inc., Princeton, New Jersey, defines the term ohmic contact as a contact between two materials possessing the property that the potential diierence across the contact is proportional to the current passing through the contact. As previously stated an ohmic Contact for N-type cadmium sulfide consists of a metal having a work function which is substantially the same or less than the work function of crystalline cadmium sulde. Illustratively ohmic contact materials are the metals gallium, indium and their equivalents. A rectifying contact for N-type cadmium sulfide is a metal which has a work function which is greater than the work function of crystalline cadmium sulfide, such illustratively as the metals copper, gold, platinum, silver and the like. Cadmium sulfide is crystallized in thin plate-like physical shape by the apparatus and the method described in patent application Number 572,170, filed March 16, 1956, by Donald C. Reynolds, the present inventor for Growth of Crystals. This application Serial Number 572,170, describes applicants means and method for growing crystals of cadmium sulfide, zinc sulfide and the like, in a hydrogen sulfide multiple temperature, oating cup furnace. The crystals are grown of a desired degree of purity. The furnace is adapted for producing both intrinsic and extrinsic crystals. A cadmium sulide crystal may have the characteristics of an extrinsic semiconductor imparted thereto for both P and N-type crystals by introducing into the furnace cadmium suliide crystal donor impurities or acceptor impurities. Shockley pages 12 to 15 and 237 and Kittel (1956) pages 347 and 353. Photovoltaic and barrier layer cells are described in Theory and Applications of Electron Tubes by Herbert I. Reich, published in 1944 4 by the McGraw Book Company, Inc. of New York city, New York, on pages 556 to` 560.

The graphs 2a, 2b and 2c of Fig. 2 bear no legends on their abscissae and ordinates in harmony with the practice in the Dunlap text, such as the Fig. 7.2 to the Fig. 8.7 from page 133 to page 153 and elsewhere. The curves presented are more the expressions of theories than they are conventionally plotted lines along a series of actual, experimentally determined measured values obtained as experimental findings. The Dunlap text at page 7 refers to an extrinsic semiconductor as a semiconductor which conducts because of the presence of impurities. Kittel (1956) at page 374 includes thermal agitation and lattice defects with impurities. Intrinsic serniconductors are those which, particularly at high temperatures,

conduct because both electrons and holes are thermally excited in pure material. In intrinsic semiconductors the impurity density is small as compared with the intrinsic carrier density at room temperature. The P-N junction is the boundary between two regions, one N-type and the other P-type. A photovoltaic cell itself produces an electric voltage when light is incident upon it.V The International Dictionary states that all pure, ideal crystal semiconductors are naturally semiconducting but the property may be very small as compared with the corresponding property in impurity semiconductors and is only to be observed at high temperatures. According to the the band theory of solids intrinsic semiconductors are described by the thermal excitation of electrons from the filled band the whole width'of the energy gap to the conduction band. An N-type semiconductor is an extrinsic semiconductor in which the conduction electron density exceeds the hole density, the implication being that the net ionized impurity concentration is of the donor type. A P-type semiconductor is an extrinsic semiconductor in which the hole density exceeds the conduction electron density with the implication that the net ionized impurity concentration is of the acceptor type. Introduction to Solid State Physics by Charles Kittel and published in 1956 by John Wiley & Sons, Inc., pages 270, 347 and 353 to 357 is concerned with deiicit semiconductors and impurity states.

Following the application of the contacts to the edges of the cadmium sulde crystal photovoltaic, the resultant cell is maintained at a temperature range of illustratively about 200 C. and to 400 C. for a period of about one to five minutes for the purpose of annealing the junction between the contact and the crystal.

A crystalline cadmium sulfide cell which is made in the described manner, consists of one contact 15 on the polished side of the crystal and a second contact 16 on the unpolished side of the crystal. Electrical energy is derived from the resultant crystal cell with its polished side under radiation from the sun by suitable means, such as by a pair of electrical energy contacting leads 17 and 18 separately attached at one of their ends to the contacts 15 and 16 and conducting electrical energy to a desired destination such as to a battery, the contacts of a galvanometer 19 or the like. A irst type of crystalline cadmium sulde cell employs the P-N junction of crystalline cadmium sulfide itself. This first type of cell consists of an N-type section of cadmium sulfide in contact with a P-type section of cadmium sulde in the same crystal over a barrier layer 13. The term barrier layer is applied to the region in a semiconductor which is practically stripped of conduction electrons. Kittel (1956) page 388. The International Dictionary delines the barrier layer as an electrical double layer formedy at the surface of contact between a metal and a semiconductor or between two metals, in order that the Fermi levels in each material should be the same. The term junction in a semiconductor device is a region of transition between semiconducting regions of different electrical properties. A diffused junction is a junction formed by the diusion of an impurity within a semiconductor crystal.

A doped junction is a junction produced by the addition of an impurity to the melt during crystal growth. A grown junction is a junction produced during growth of a crystal from a melt. An N-N junction is a region of transition between two regions having different properties in N-type semiconducting material. A P-P junction corresponds in P-type semiconductor material. A P-N junction is a region of transition between P- and N-type semiconducting material. Both contacts 15 and 16 of this first type of cell are ohmic contacts.

A second type of crystalline cadmium sulfide cell employs on the polished N-type side of the cadmium sulfide crystal an ohmic contact 15, made of a metal such as indium, gallium or the like. On the unpolished side of the cadmium sulfide crystal is adhered a rectifying contact 16, made of a metal having a work function which is greater than the work function of cadmium sulfide, such illustratively as the metals copper, gold, platinum, silver and the like.

A thrid type of crystalline cadmium sulfide cell of the P-type has bonded to the edge of its polished side 11 an ohmic contact 15. On the unpolished side 12 of the cadmium sulfide crystal is adhered a rectifying contact made of a metal or an alloy having a work function, which is less than the Work function of cadmium sulfide, of which illustratively are the metals indium, gallium, etc.

Modifications of the photovoltaic cell shown in Figs. l and 2 of the drawings are shown in Figs. 5 and 6 of the drawings. In Fig. 5 an N-type cadmium sulfide crys-tal 20 has an ohmic edge contact 21 of indium or gallium along the edge of one surface and its opposite surface is covered with a layer of metal 22, such as copper or silver. In Fig. 6 a P-type cadmium sulfide crystal 23 has an edge Contact 24 along the edge of one surface and its opposite surface is covered with a layer of metal 25 of indium or gallium.

In Fig. 7 of the accompanying drawings is shown a composite type of cell which is within the concept of the present invention. One composite cell comprises an N-type of cadmium sulfide crystal 26 in contact with a P-type of semiconductor 27. The P-type semiconductor 27 may consist illustratively of cuprous oxide, cuprous sulfide, cupric sulfide, selenium, and the like. Contacts 28 and 29 are applied along the edges of both sides of the cell. The electrical contacts 28 and 29 are of the ohmic type with work functions the same or less than the work function of cadmium sulfide. The elements indium and gallium are representative of contacts of the ohmic type. Illustratively the contacts 28 and 29 may be made of indium or of gallium.

Another composite cell comprises a P-type of cadmium sulfide crystal in contact with an N-type of semiconductor. N-type semiconductors illustratively consist of germanium, silicon and the like. Ohmic contacts are used on both sides of both types of composite cells.

In Fig. 8 of the accompanying drawings is shown a spectral response curve 30 of a cadmium sulfide photo- -voltaic cell, such as the cells disclosed herein, superimposed upon the solar spectrum curve 31. The coordinates of the curves 30 and 31 are in wave lengths along the absciss'a and in current along the ordinate. The cell response curve is characterized by two peaks 32 and 33, the earlier peak 32 being at the cell absorption cutoff. At the second peak 33 each photon from the sun produces an electron. Electrons so produced leave the cell and are recordable on the galvanometer 19 or on other recording instruments.

The photovoltaic cells which are shown and described herein have been submitted as successfully operative embodiments of the present invention and are intended to include structural and functional equivalents and modifications thereof.

What I claim is:

l. A composite barrier layer photovoltaic cell for exposure to solar spectrum energy as a source of electrical energy derived from across the photovoltaic cell comprising an N-type cadmium sulfide crystal with an impurity atom incorporated in its crystal lattice, a P-type semiconductor selected from the group consisting of cuprous oxide, cuprous sulfide, cupric sulfide and selenium in contact with the cadmium sulfide crystal, a first ohmic contact selected from the group consisting of indium and gallium secured to the side of the cadmium sulfide crystal remote from the semiconductor, and a second ohmic contact selected from the group consisting of indium and gallium secured to the side of the semiconductor remote from the cadmium sulfide crystal.

2. A composite barrier layer photovoltaic cell for eX- posure to solar spectrum energy as a source of electrical energy derived from across the photovoltaic cell comprising a P-type cadmium sulfide crystal with an impurity atom incorporated in its lattice, an N-type semiconductor selected from the group consisting of cuprous oxide, cuprous sulfide, cupric sulfide and selenium in contact with the P-type cadmium sulfide crystal, and ohmic contacts selected from the group consisting of indium yand gallium on the opposite sides of the cell at which contacts opposite potentials appear derived from across the photovoltaic cell as a conversion of solar spectrum energy into electrical energy..

3. A barrier layer cell comprising N-type cadmium sulfide, and a P-type semiconductor selected from the group consisting of cuprous oxide, cuprous sulfide, cupric sulfide and selenium in contact with the N-type cadmium sulde.

4. A cadmium sulfide composite cell consisting of an N-type cadmium sulde crystal, and a P-type semiconductor contacting the N-type cadmium sulde crystal and wherein the P-type semiconductor consists of a material selected from the group: cuprous oxide, cuprous sulfide, cupric sulfide and selenium, 'and inclusive of a first electrical contact attached to the cadmium sulde crystal, and a second electrical contact attached to the semiconductor for making available the continuous withdrawal of electrical energy from the two contacts when solar energy is applied to the cell.

References Cited in the file of this patent UNITED STATES PATENTS 2,582,850 Rose Jan. 15, 1952 2,622,117 Benzer Dec. 16, 1952 2,688,564 Forgue Sept. 7, 1954 2,736,848 Rose Feb. 28, 1956 OTHER REFERENCES Reynolds: D. C., Photovoltaic Effect in Cadmium Sulfide, The Physical Review, 96, S33-534, Oct. 1954. 

4. A CADMIUM SULFIDE COMPOSITE CELL CONSISTING OF AN N-TYPE CADMIUM SULFIDE CRYSTAL, AND A P-TYPE SEMICONDUCTOR CONTACTING THE N-TYPE CADMIUM SULFIDE CRYSTAL AND WHEREIN THE P-TYPE SEMICONDUCTOR CONSISTS OF A MATERIAL SELECTED FROM THE GROUP: CUPROUS OXIDE, CUPROUS SULFIDE, CUPRIC SULFIDE AND SELENIUM, AND INCLUSIVE OF A FIRST ELECTRICAL CONTACT ATTACHED TO THE CADMIUM SULFIDE CRYSTAL, AND A SECOND ELECTRICAL CONTACT ATTACHED TO THE SEMICONDUCTOR FOR MAKING AVAILABLE THE CONTINUOUS WITHDRAWAL OF ELECTRICAL ENERGY FROM THE TWO CONTACTS WHEN SOLAR ENERGY IS APPLIED TO THE CELL. 